Possible the heads were starved of oil briefly and that was the rattle I heard but like I said I've done 700 miles and its running 100% oil pressure is good and there are no noises at all now

 

Oh my god, only just seen this! Feel so sorry for you Craig after only just buying it.

I know everything seems well, but honestly for what little knowledge I have that metal to metal clacking for even a brief period must have caused some damage, no matter how small. OK so it has new oil filling in the gaps at the moment but if I were you I'd start putting some money aside and buy a spare bottom end even if it's a case of it sitting in your garage.
Sorry to put a dampener on things but as the experts have identified the sound as possible bottom end failure then they know best, and its probably best to plan for the worst, and if the problem never arises you have a spare engine to re-sell.

Are you confident when using the car or are you cautious in expectation?

 

Way ahead of u mate have already started putting money aside for the worst I'm confident with it however I'm taking it back at the end of the month for a oil and filter change and inspect the oil for signs of metal in the oil etc

 

Good luck pal, hope it was top end noise like you said